[EE] Selecting a stepper motor

[David C Brown]

I am attempting to design a solar panel positioning system for use on a
small canal boat. This will obviously use an alt-az positioner and the
azimuth is straight forward but the altitude is somewhat more difficult.

The obvious solution of mounting the panel in bearings at its centre of
gravity and driving the axle would have the panel too far above deck when
stowed at zero altitude.

So I cam up with the solution shown in the attached drawing. The panel is
hinged at its lower edge and a motor behind the panel drives an arm which
bears against the back of the panel.

My problem now is I can't for the life of me calculate the torque needed
from the motor. Could some kind soul point me in the right direction please
?
[image: Capture.JPG]
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



*Sent from my etch-a-sketch*

[Neil]

Math is all just vectors (which I can help you with), but the torque
will be uneven through its the travel, so what's the range of angles
this needs to travel through?
However, I'm not seeing the benefit to this style of lifting. Some
quick alternate thoughts...

- How about adding a small but heavy counterweight extend out the left
side of the panel? Any weight would help if not the full weight.
- You can rotate the panel with a gear/pulley attached to the rotation
point, and a gear of smaller size at the motor to increase torque. This
can be timing belt or sprocket/chain.
- Depending on how much tilt you need (and the speed you need), you
might consider a linear actuator pushing up on the panel... acme
threaded rod/bearing.

Cheers,
-Neil.

Post by David C Brown
I am attempting to design a solar panel positioning system for use on a
small canal boat. This will obviously use an alt-az positioner and the
azimuth is straight forward but the altitude is somewhat more difficult.
The obvious solution of mounting the panel in bearings at its centre of
gravity and driving the axle would have the panel too far above deck when
stowed at zero altitude.
So I cam up with the solution shown in the attached drawing. The panel is
hinged at its lower edge and a motor behind the panel drives an arm which
bears against the back of the panel.
My problem now is I can't for the life of me calculate the torque needed
from the motor. Could some kind soul point me in the right direction please
?
[image: Capture.JPG]
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>
*Sent from my etch-a-sketch*

[Denny Esterline]

Approaching this with a full analytic solution is probably pointless. Even
with static/dynamic analysis of the
mass of the panel, the wind, rain, snow, someone leaning on it, pitch of
the craft, etc all add enough
uncertainty that the numbers are near enough irrelevant.
As a first order guess, take the mass of the panel and assume it a point
load on the end of the drive arm
and assume the arm to be horizontal - all worst case scenarios.

You probably don't want the stepper arrangement to be exactly as you've
sketched it. In this config, the
motor would constantly need to be powered to fight gravity. That motor load
would be a significant portion
of the power from the panel. Running the same motor through a leadscrew or
possibly a worm drive
mechanism would allow the motor to be powered off and the panel remain
stationary.

Also, I probably wouldn't recommend a stepper for this application.
Steppers are great for open-loop
positioning when the motor can be sized significantly above the load to
assure no missed steps or
back-driving. Further, it's a solution I would look at for a ground station
trying to base the position on
emeritus data - not so much a mobile unit unless you want to figure GPS
position as well as roll/pitch/yaw
into the calculations. In your application I would consider two (per axis)
small solar cells or possibly LDRs
set on opposite sides of a baffle. Mount this so it moves with the panel.
Then set the control system to
drive the angle such that the two sensors have equal output. Add a fairly
long time constant. (day-night
cycle is 1/4 degree per minute)
This web site is a fairly poor design, but the graphics should help explain
the concept.
https://www.pc-control.co.uk/howto_tracksun.htm

-Denny

Post by David C Brown
I am attempting to design a solar panel positioning system for use on a
small canal boat. This will obviously use an alt-az positioner and the
azimuth is straight forward but the altitude is somewhat more difficult.
The obvious solution of mounting the panel in bearings at its centre of
gravity and driving the axle would have the panel too far above deck when
stowed at zero altitude.
So I cam up with the solution shown in the attached drawing. The panel is
hinged at its lower edge and a motor behind the panel drives an arm which
bears against the back of the panel.
My problem now is I can't for the life of me calculate the torque needed
from the motor. Could some kind soul point me in the right direction please
?
[image: Capture.JPG]
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>
*Sent from my etch-a-sketch*
--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

[David C Brown]

Well you have convinced me that a lead screw is a better arrangement.

The problem with a close loop design is keeping the sensors clear of
obstruction, especially the azimuth ones which would be very low on the
deck and easily obscured by ropes, buckets etc.

Whilst canal boats are mobile they move very slowly and are subject to
minimal pitch and roll. Yaw would be taken care by an electronic compass.
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



*Sent from my etch-a-sketch*

Post by Denny Esterline
Approaching this with a full analytic solution is probably pointless. Even
with static/dynamic analysis of the
mass of the panel, the wind, rain, snow, someone leaning on it, pitch of
the craft, etc all add enough
uncertainty that the numbers are near enough irrelevant.
As a first order guess, take the mass of the panel and assume it a point
load on the end of the drive arm
and assume the arm to be horizontal - all worst case scenarios.
You probably don't want the stepper arrangement to be exactly as you've
sketched it. In this config, the
motor would constantly need to be powered to fight gravity. That motor load
would be a significant portion
of the power from the panel. Running the same motor through a leadscrew or
possibly a worm drive
mechanism would allow the motor to be powered off and the panel remain
stationary.
Also, I probably wouldn't recommend a stepper for this application.
Steppers are great for open-loop
positioning when the motor can be sized significantly above the load to
assure no missed steps or
back-driving. Further, it's a solution I would look at for a ground station
trying to base the position on
emeritus data - not so much a mobile unit unless you want to figure GPS
position as well as roll/pitch/yaw
into the calculations. In your application I would consider two (per axis)
small solar cells or possibly LDRs
set on opposite sides of a baffle. Mount this so it moves with the panel.
Then set the control system to
drive the angle such that the two sensors have equal output. Add a fairly
long time constant. (day-night
cycle is 1/4 degree per minute)
This web site is a fairly poor design, but the graphics should help explain
the concept.
https://www.pc-control.co.uk/howto_tracksun.htm
-Denny

Post by David C Brown
I am attempting to design a solar panel positioning system for use on a
small canal boat. This will obviously use an alt-az positioner and the
azimuth is straight forward but the altitude is somewhat more difficult.
The obvious solution of mounting the panel in bearings at its centre of
gravity and driving the axle would have the panel too far above deck when
stowed at zero altitude.
So I cam up with the solution shown in the attached drawing. The panel

is

Post by David C Brown
hinged at its lower edge and a motor behind the panel drives an arm which
bears against the back of the panel.
My problem now is I can't for the life of me calculate the torque needed
from the motor. Could some kind soul point me in the right direction please
?
[image: Capture.JPG]
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>
*Sent from my etch-a-sketch*
--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist